The present invention relates to runflat pneumatic tires for motor vehicles and more specifically, to bead-region chipper-type reinforcements to improve riding comfort and tire durability at high speeds and during runflat operation.
Modern pneumatic vehicle tires typically include a pair of axially separated, inextensible beads which serve to hold the tire on a wheel rim as well as to provide a structural foundation for the reinforced carcass plies to which the other tire components, such as the sidewalls and tread, are attached. A circumferentially disposed bead filler apex extends radially outward from each of the two beads.
One or more carcass plies extend between the two beads, by way of the sidewalls and the tire crown. Each carcass ply has two axially opposite end portions. The end portions of at least one carcass ply are turned up around, or clamped to, the beads, thereby anchoring the ends of one or more carcass ply layers. During tire construction, tread rubber and sidewall rubber are applied axially and radially outward of the one or more reinforced carcass plies.
The bead region of the sidewall is one part of the tire that contributes a substantial amount to the rolling resistance or rolling friction of the tire. The rolling resistance corresponds to an energy loss taking place within the tire""s structure and is related to the cyclical flexure of the tire components, including the tread and its underlying structures such as the belts, as well as, especially, the portions of the sidewall that are closest to the bead regions where the flexural strain, and energy loss, is greatest.
The energy losses associated with a tire""s rolling resistance correspond to heat accumulation within the tire""s structure. Under conditions of severe operation, as at high speed or during operation of an uninflated runflat tire having extended mobility properties, flexure-induced heating in the bead region portion of the sidewalls can be especially problematic.
U.S. Pat. No. 3,253,693 discloses data on radial and circumferential deformations within tires. The deformations that take place in the bead region of the sidewalls result in shearing stresses during normal operation of the tire and especially during severe operating conditions. Bead-region shear stresses have both circumferential and radial orientation. The resulting shear strains or deformations correlate with high flexure within the bead regions. In the case of runflat or extended mobility technology (EMT) tires, sidewall flexure in the bead region can lead to a shortened runflat operational service life. More specifically, EMT tires operating under runflat conditions inevitably undergo deterioration and failure after operation for a certain distance; often the failure mode involves complete cracking of the parts of the tire (i.e., the chafers) that make the most immediate contact with the steel wheel rim""s radially extending edge. The chafer cracks are oriented at 45 degrees indicating a shear strain effect in the bead regions.
Recent investigations have shown a high difference of radial-circumferentially oriented shear strains between the footprint area and the part of the tire that is 180 degrees from the footprint, i.e., the top of tire. This difference between the shear strains at the top and bottom of the tire is also referred to as the cycle amplitude of shearing strain, a variable which, when extreme, correlates with chafer cracking during the uninflated operation of EMT tires.
Among the methods used to reinforce the bead region of radial-ply tires is the incorporation of xe2x80x9cchippers.xe2x80x9d A chipper is a circumferentially deployed metal or fabric layer that is disposed within the bead region in the portion of the tire where the bead fits onto the wheel rim. More specifically, each of the chipper(s)(one or more) used in each bead region of a given tire typically lies inward of the wheel rim (i.e., toward the bead) and inward (i.e., radially inward, relative to the bead viewed in cross section) of the portion of the ply that turns upward around the bead. Typical single chippers are made of parallel-aligned, metal or polymer cords that are oriented at an angle of 25 degrees with respect to the circumferential direction.
The width of the chipper is the distance to which it extends radially outward from the bead region. The width of the chipper is one variable that can be used to xe2x80x9ctunexe2x80x9d a tire""s handling and steering performance. Chippers typically extend to a radial distance of to about 20 millimeters above the wheel""s rim flange.
Generally, chippers provide a stiffening influence to the radially inward portion of the sidewall most adjacent to the bead region. The stiffening increases the resistance to cyclical flexure of the sort referred to above. In other words, the increased stiffness afforded by chippers works to reduce the amount of flexural deformation and resultant shearing stresses and strains in the axially inward portions of the sidewalls that are most immediately adjacent to the beads.
The use of wire chippers in standard non-EMT tires improves handling and steering performance, especially at high speeds. The formation of standing waves in non-EMT tires during high-speed operation can also be inhibited by the stiffness/damping characteristics of the final tire design, including the choice of chipper width. Flatspotting, i.e. the tendency of the tread of a tire to sustain a flat spot in the ground-contacting portion of the tread when a vehicle has been parked or otherwise sitting for a prolonged period, is also alleviated by the use of chippers.
A balanced design for a chipper-reinforced bead assembly of a tire would include stress characteristics that lead to reduced flexural energy generation (heat buildup) and to strain characteristics that can be uniformly borne by mutually adjacent tire components in the bead region. The objective of a balanced design is to achieve high-speed handling and steering benefits without compromising riding comfort due to the increased rigidity associated with typical chipper designs.
It is an object of the present invention to provide an optimized chipper design for use in a runflat radial tire as defined in one or more of the appended claims and, as such, having the capability of accomplishing one or more of the following subsidiary objects.
One object of the present invention is to provide an improved chipper design that minimizes the shear strain cycle amplitude in the bead region of runflat tires during runflat operation in order to minimize the formation of cracks in the chafer region.
Another object of the present invention is to provide an improved chipper design that reduces the flatspotting tendency of a runflat tire during normal-inflated service.
Yet another object of the present invention is to provide an improved chipper design that serves to minimize the potential for the formation of standing waves during high-speed, normal-inflated operation.
And another object of the present invention is to provide an improved chipper design that contributes to improved vehicle comfort and handling during normal-inflated operation while contributing to the tire""s runflat durability.
The present invention relates to a pneumatic runflat radial ply tire having a tread, a carcass comprising at least a radial ply, a belt structure located between the tread and the radial ply, two inextensible beads, and two sidewalls with inserts. The respective bead regions of the runflat tire are reinforced with circumferentially disposed chippers in order to minimize the formation of cracks in the chafer region caused by runflat operations. Each chipper contains reinforcing cords (preferably wires) that are oriented at an angle of between 30xc2x0 and 50xc2x0 in the circumferential direction. The radially outermost end of each chipper is located between 5% and 20% of the section height above the wheel rim flange of the wheel-mounted tire. And the chipper is disposed axially inward of the turnup end of the turned up ply.
An alternative chipper design is positioned and sized similarly, but comprises two layers of crossed reinforcement cords.